The Collaborative International Dictionary of English v.0.48:

Speculate \Spec"u*late\, v. t.
   To consider attentively; as, to speculate the nature of a
   thing. [R.] --Sir W. Hamilton.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Speculate \Spec"u*late\, v. i. [imp. & p. p. Speculated; p.
   pr. & vb. n. Speculating.] [L. speculatus, p. p. of
   speculari to spy out, observe, fr. specula a lookout, fr.
   specere to look. See Spy.]
   1. To consider by turning a subject in the mind, and viewing
      it in its different aspects and relations; to meditate; to
      contemplate; to theorize; as, to speculate on questions in
      religion; to speculate on political events.
      [1913 Webster]

            It is remarkable that persons who speculate the most
            boldly often conform with the most pefect quietude
            to the external regulations of society. --Hawthorne.
      [1913 Webster]

   2. (Philos.) To view subjects from certain premises given or
      assumed, and infer conclusions respecting them a priori.
      [1913 Webster]

   3. (Com.) To purchase with the expectation of a contingent
      advance in value, and a consequent sale at a profit; --
      often, in a somewhat depreciative sense, of unsound or
      hazardous transactions; as, to speculate in coffee, in
      sugar, or in bank stock.
      [1913 Webster]

WordNet (r) 3.0 (2006):

speculate
    v 1: to believe especially on uncertain or tentative grounds;
         "Scientists supposed that large dinosaurs lived in swamps"
         [syn: speculate, theorize, theorise, conjecture,
         hypothesize, hypothesise, hypothecate, suppose]
    2: talk over conjecturally, or review in an idle or casual way
       and with an element of doubt or without sufficient reason to
       reach a conclusion; "We were speculating whether the
       President had to resign after the scandal"
    3: reflect deeply on a subject; "I mulled over the events of the
       afternoon"; "philosophers have speculated on the question of
       God for thousands of years"; "The scientist must stop to
       observe and start to excogitate" [syn: chew over, think
       over, meditate, ponder, excogitate, contemplate,
       muse, reflect, mull, mull over, ruminate,
       speculate]
    4: invest at a risk; "I bought this house not because I want to
       live in it but to sell it later at a good price, so I am
       speculating" [syn: speculate, job]